INACTIVATION OF THE ACCEPTOR SIDE AND DEGRADATION OF THE D1 PROTEIN OF PHOTOSYSTEM II BY SINGLET OXYGEN PHOTOGENERATED FROM THE OUTSIDE

Abstract

Abstract— The possible association of photodynamic sensitization with photoinhibition damage to the photosystem II complex (PS II) has been investigated using isolated intact thylakoids from pea leaves. For this study singlet oxygen (1O2), photoproduced by endogenous chromophores that are independent of the function of PS II, was assumed to be the major reactive intermediate involved in the photoinhibition process. When thylakoid samples preincubated with rose bengal were subjected to exposure to relatively weak green light (500–600 nm) under aerobic conditions, PS II was severely damaged. The pattern of the rose bengal‐sensitized inhibition of PS II was similar to that of high light‐induced damage to PS II: (1) the secondary quinone (QB)‐dependent electron transfer through PS II is inactivated much faster than the QB‐independent electron flow, (2) PS II activity is lost prior to degradation of the D1 protein, (3) diuron, an herbicide that binds to the QB domain on the D1 protein, prevents D1 degradation, and (4) PS II is damaged to a greater extent by the deuteration of thylakoid suspensions but to a lesser extent by the presence of histidine. Furthermore, it was observed that destroying thylakoid Fe‐S centers resulted in a marked reduction of high light‐induced PS II damage. These results may suggest that the primary processes of photoinhibition are mediated by 1O2 and that Fe‐S centers, which are located in some membrane components, but not in PS II, play an important role in photogenerating the activated oxygen immediately responsible for the initiation of photodamage to PS II.